Chromosomal deletions at 11q21-23 have recently been reported to be common aberrations in mantle cell lymphoma (MCL). To characterize the structure of the deletion, we studied 41 cases of MCL by fluorescence in situ hybridization using a YAC contig, which spans the region at 11q22.1-23.3. 17 MCLs were studied using a set of 20 yeast artificial chromosomes (YACs) in a contig, and nine of these cases showed deletion of 11q22-23. The deletion spanned several megabases in all but one case, where only YAC 755b11 at 11q23.1, covering approximately a 1.6 Mb of DNA, was deleted. Analysis of additional 24 MCLs with YAC 755b11 revealed the deletion in 49% of all cases (20/41). The deleted region at 11q22.1-23.3 was discontinuous in five lymphomas and in the majority of the cases the distal breakpoint occurred between YACs 785e12 and 911f2 at 11q23.3. We conclude that the deletion of 11q22-23 and particularly the deletion of YAC 755b11 are very common in MCL and may be important in the genesis or progression of the disease.